A tire for carrying heavy load such as a truck or a bus, the use of a so-called tubeless tire is spreading widely. In such a tubeless tire, as compared with the general tube type tire, such tubeless tire is joined to the rim with a strong bond. Therefore, when a load is applied to the tire, as shown in FIG. 6, a larger bending force occurs in the bead part (a), causing a tensile force inside and a stress of compression outside. When the carcass (b) is exposed to such bending force, the turned up portion (c) of the carcass (b) is pulled toward the main portion (e) side of the carcass (b), like both ends of a rope on a pulley, around the bead core (d), and is dislocated.
To prevent such movement of the carcass (b), hitherto, as shown in FIG. 5 (b), the front end of the turned up portion (c) of the carcass (b) is positioned on the bending neutral line of the bead part (a), and the carcass (b) is reinforced by covering the outside of the carcass (b) with a filler (f) made of organic fiber cords. On the other hand, prior arts of the reinforcing structure of the 15 deg. taper bead are disclosed in the Japanese Laid-open Patent Nos. 60-94806, 54-13108, and the Japanese Patent Publication No. 60-6803, among others, and various proposals have been made for the appropriate arrangement method of nylon filler and steel filler, suited hardness of apex rubber, and other proposals.
In this case, when reinforcing the turned up portion of the carcass ply with the nylon filler alone, the strength is insufficient with one filler, therefore two to four fillers are overlaid to achieve a desired reinforcement effect (FIG. 5a). Or when using a steel filler, a certain reinforcement effect is obtained when using one filler. Therefore, usually one filler is used (FIG. 5c), or in order to further enhance the reinforcement effect, the combination of two nylon fillers and one steel filler is most widely employed.
Moreover, according to the Japanese Laid-open Patent No. 63-87304, it is proposed to dispose a first reinforcement layer of steel filler between the bead apex rubber and carcass main portion, and a second reinforcement layer of steel filler overlapping with the first reinforcement layer through the carcass main portion, at the inner side of the tire as shown in FIG. 5(d).about.(f).
Moreover, in the Japanese Laid-open Patent No. 60-94806, a rubber band of high elasticity is employed between the carcass main portion and a bead apex rubber. This is based on the consideration that as the tire is deformed in a convex form projecting to the outside in the axial direction of the tire from the side wall to the bead part when loaded, the bead upper part is deformed flexibly so as to overhang the rim flange part. And in this time, the cord gap (the gap between two adjacent cords) of the carcass main portion is forced to be extended. In cooperation with this movement, the turned up portion of the carcass ply and the steel filler reinforcement layer are pushed to the outside in the tire axial direction. As this flexing is repeated periodically, the temperature of the bead part rises. That is, the rubber of the bead part is always exposed to dynamic and thermal fatigue. Accordingly, as the means for preventing widening of the cord gap of the carcass ply main portion and restricting the temperature rise of the bead part, in the Japanese Laid-open Patent No. 60-94806, the "rubber band of high elasticity" is employed at the outside of the carcass main portion in the tire axial direction, that is, at the boundary of the bead apex. Such employment suppresses the widening of the cord gap. And using a rubber layer of a small hysteresis loss in the bead apex inhibits heat generation.
However, in the method of reinforcement of the bead part by nylon fillers alone, the number of fillers increases, so that the weight increases and productivity is restricted.
And along with the increase of the wall thickness of the bead part, there is a limit due to heat generation of the tire arising from running.
The reinforcement of the bead part by the combination of steel filler and nylon filler is also limited in the improvement of durability because of the increase of weight and the problem in productivity.
When only the steel filler is used, a certain reinforcement effect is obtained by one filler and there is no problem in weight and productivity. But the bead part is tilted toward the outer side in the tire axial direction when inflated with internal pressure and when loaded. As the result, strain concentration increases at the terminus of turnup of steel filler or carcass ply. As a result, separation from the surrounding rubber occurs, and notable improvement of durability is not expected.
Accordingly, to prevent tilting of the bead part, as shown in FIG. 5 (c), it is intended to enhance the durability by restricting the movement of the bead part when inflated with internal pressure and when loaded, by raising the height HE of the upper terminal SE of the inside of the tire of the steel filler SF. But this means is also limited because if the height HE of the upper terminal SE is raised too much, a crack may be formed at the tire inside upper terminal SE of the steel filler SF and this crack may propagate into the inner liner to allow air to invade, finally leading to problems.
Among the aforementioned method of reinforcement for sandwiching the carcass main portion from both side with the steel filler at the bead part (the Japanese Laid-open Patent No. 63-87304), in the method shown in FIG. 5 (d), since the turned up portion (c) of the carcass ply is not reinforced by the steel filler, problems due to rim deviation are likely to occur. In the structure shown in FIG. 5 (e), there is a loop arrangement of the second reinforcement layer SF-2 from axially inside to outside of the bead part. In this case, rim deviation may be prevented, but the inside region of the second reinforcement layer SF-2 is pulled to the lower side in the radial direction, while the adjacent carcass ply main body portion (e) is pulled to the upper side in the radial direction. Therefore shearing strain is likely to occur at the interface of the two, thereby resulting in peeling trouble between the second reinforcement layer SF-2 and the carcass ply main portion (e). In the structure in FIG. 5 (f), rim chafing damage is likely to be induced at the bead base side.
Furthermore, in the structure disclosed in the Japanese Laid-open Patent No. 60-94806, it is expected to enhance the durability more than the conventional tire, but in the "rubber band with high elasticity," widening of the cord gap at high load cannot be suppressed sufficiently.